Modern electro-optical applications are demanding relatively inexpensive, miniaturized lasers capable of producing a series of well-defined output pulses. As such, a variety of microlasers have been developed which include a microresonator and a pair of at least partially reflective mirrors disposed at opposite ends of the microresonator to define a resonant cavity therebetween. The microresonator of one advantageous microlaser includes an active gain medium and a saturable absorber that serves as a Q-switch. See, for example, U.S. Pat. No. 5,394,413 to John J. Zayhowski which issued on Feb. 28, 1995, the contents of which are incorporated in their entirety herein. By appropriately pumping the active gain medium, such as with a laser diode, the microresonator will emit a series of pulses having a predetermined wavelength, pulse width and pulse energy.
As known to those skilled in the art, the energy of pulses emitted by a microlaser is dependent upon the materials from which the active gain medium and the saturable absorber are formed as well as the length and width of the optically pumped volume. All other factors being equal, the longer and wider the microresonator cavity, the greater the pulse energy and average power of the resulting laser pulses.
Conventional microlasers, such as those described by U.S. Pat. No. 5,394,413, are end pumped in a direction parallel to the longitudinal beam axis defined by the resonator cavity. In this regard, the longitudinal axis of the microresonator cavity extends lengthwise through the resonator cavity and is oriented so as to be orthogonal to the pair of at least partially reflective mirrors that define the opposed ends of the resonant cavity. As such, conventional microlasers are configured such that the pump source provides pump input in a direction perpendicular to the at least partially reflective mirrors that define the opposed ends of the resonant cavity. The effective length of the resonator cavity is therefore equal to the thickness of the monolithic medium comprising the resonator.
While the microlaser can be fabricated such that the resonator cavity has different lengths, a number of factors contribute to generally limit the permissible length of the resonator cavity. See, for example, U.S. Pat. No. 5,394,413 that states that the resonator cavity, including both the saturable absorber and the gain medium, is preferably less than two millimeters in length. In particular, a number of electrooptical applications require microlasers that are extremely small. As such, increases in the length of the resonator cavity are strongly discouraged in these applications since any such increases in the length of the resonator cavity would correspondingly increase the overall size of the microlaser. Additionally, it is difficult to fabricate monolithic microlasers longer than a few millimeters in length because of limitations in material growth processes, which would produce insufficiently uniform optical media to provide efficient output of good divergence and spectral purity.
Increasing the width of the resonator cavity, as might be achieved by focusing the pump source to a larger spot in the microresonator, would also be very undesirable because it would result in highly divergent output. This disadvantageous result is because a width increase will correspondingly increase the resonator Fresnel number, allowing multiple transverse modes to oscillate.
The relatively small size of conventional microlasers also limits the effectiveness with which heat generated by absorption of pump diode radiation can be removed. In certain instances, the heat generated within the microlaser may even exceed the thermal capacity of the heat sink or other heat removal device, thereby potentially distorting the medium and causing unacceptable degradation of the output beam power or quality.
The foregoing examples of practical limitations on the length of the resonator cavity also disadvantageously limit the pulse energy and average power of the pulses output by the conventional microlasers. The pulse energy and average power of the pulses emitted by a microlaser are also dependent upon the power level at which the active gain medium pumped, i.e., the power delivered by the pump inputs. Since conventional microlasers are end pumped, a single stripe laser diode is typically utilized as the pump source since the pump input generated by a single stripe laser diode generally fill the relatively small microlaser mode diameter. Unfortunately, the power delivered by a single stripe laser diode is typically limited to about 1 to 3 watts, thereby correspondingly limiting the pulse energy and average power of the pulses output by a conventional microlaser. In addition, multi-stripe diodes are generally not utilized to end pump a conventional microlaser since multi-stripe diodes focus poorly and, as a result, require complex optics to produce a small enough spot to generate single mode microlaser output.
In contrast, some modem electro-optical applications are beginning to require microlasers that emit pulses having greater pulse widths, such as pulse widths of greater than 1 nanosecond and, in some instances, up to 10 nanoseconds, as well as pulses that have greater pulse energy, such as between about 10 .mu.J and about 100 .mu.J, and greater average power, such as between 0.1 watts and 1 watt. As a result of the foregoing limitations on the length of the resonator cavity and the corresponding limitations on the pulse widths, pulse energy and average power of the pulses output by the conventional microlasers, conventional microlasers do not appear capable of meeting these increased demands.